Field of the Invention
The invention relates to a system for contactless power and data transmission between a mobile station and a stationary station, in which the mobile station has a tunable resonant circuit that is driven by a control circuit, the control circuit has a multiplicity of capacitors which can be switched in in parallel with the resonant circuit by a switching device, and a device is provided for driving the switching device.
Such systems generally include a stationary station and a mobile part, for example in the form of a smart card or a chip key, etc.. In that case the stationary station provides the power for the mobile part. A loosely coupled coil pair, wherein each coil is assigned to a resonant circuit in the mobile part and the stationary part, is generally provided for that purpose. The power transmission is effected in that case by a radio-frequency signal which is fed to a rectifier nit and a downstream regulating unit in the mobile part.
In the case of such systems with contactless power transmission, the efficiency is highest when the transmission frequency of the primary circuit corresponds to the resonant frequency of the secondary circuit. For example, the primary circuit in that case constitutes the resonant circuit of the stationary part, and the secondary circuit that of the mobile part. Due to component fluctuations, that requirement can only be achieved within the sum of component tolerances. The technical requirements of power loss, quality of the components, coupling factor of primary and secondary circuits, etc., necessitate tuning of the secondary circuit.
German Patent DE 38 10 702 C2 specifies a device for contactless power and data transmission in which the stationary station contains a tunable resonant circuit. The latter is driven by a control circuit in order to carry out tuning to a resonant circuit in the mobile part, thereby achieving the best possible reception. For the purpose of tuning, variable-capacitance diodes are connected in parallel with the resonant circuit of the stationary part and are controlled by a value-continuous control voltage derived from a regulating circuit. The variable-capacitance diodes are readjusted by altering the control voltage, thereby counteracting detuning of the circuit.